how to get the last column used/last column that has some data in a excel file programmatically by using C#?

You can easily achieve this by just adding the three lines to your code as shown and a method to get your column name. So your final code will be similar to the following code.

List<string> myValue = new List<string>();
Range usedRange = sheet.UsedRange;
int lastUsedcolumn = usedRange.Column + usedRange.Columns.Count - 1;
string Column = GetExcelColumnName(lastUsedcolumn);
//In the above line you have got the last column name
//In the following line you will be appending your row number 
string endRange= Column +1; 
  foreach (Range row in sheet.Cells.Rows.Range["A1", endRange])
      {
        Range cell = (Range)row.Cells[1, 1];
          if (cell.Value2 != null)
             {
                myValue.Add(Convert.ToString(cell.Value2));
             }
      }

public string GetExcelColumnName(int columnNumber)
        {
            int num1 = columnNumber;
            string str = string.Empty;
            int num2;
            for (; num1 > 0; num1 = (num1 - num2) / 26)
            {
                num2 = (num1 - 1) % 26;
                str = Convert.ToChar(65 + num2).ToString() + str;
            }
            return str;
        }